Update zkapauthorize to use mach-nix and nixos-21.05.

docs/dev/README.rst

@@ -62,7 +62,7 @@ To update the version of packages we import from gitlab, run:
 
 .. code: shell
 
-   nix-shell --command 'tools/update-gitlab nixos/pkgs/<package>/repo.json'
+   nix-shell --command 'update-gitlab-repo nixos/pkgs/<package>/repo.json'
 
 That will update the package to point at the latest version of the project.\
 The command uses branch and repository owner specified in the ``repo.json`` file,